As described for example in U.S. Pat. No. 7,008,979; fog formation occurs under conditions of high humidity and high temperature or at interfacial boundaries where there is a large temperature and humidity difference. Coatings which reportedly reduce the tendency for surfaces to “fog up” (i.e., anti-fogging coatings) have been suggested.
In order to prevent this fogging, it is known to use various surface active agents to provide anti-fog properties to articles. For example, hydrophilic agents have been added to polyurethanes in order to impart anti-fog properties. Anti-fog coating compositions for transparent surfaces which include a three-dimensional cross-linked polyurethane having a free surface active agent disposed within open domains in its cross-linked structure have been suggested. The coating compositions are prepared by reacting isocyanates with polyfunctional polyols to obtain a polyurethane, and subsequently contacting the thus prepared polyurethane with a hydrophilic surface-active agent in order to diffuse molecules of the surface-active agent into the interior of the coating. (See for example U.S. Pat. Nos. 4,551,484 and 4,609,688 to Radisch et al.)
The surface-active agent, however, is not chemically reacted into the polyurethane, but is instead physically disposed within the polymeric structure. As such, the cured coating is susceptible to undesirable leaching and erosion of the surfactant, thereby decreasing the anti-fog properties of the coating composition.
It has also been proposed to react surface active agents into a polyurethane coating composition in order to impart anti-fog properties to the coating composition. For example, the addition of sulfonated “resins” to polyurethanes in order to prepare coatings with various properties including anti-fog characteristics have been suggested. The resins are prepared from diols or diamines reacted with di-carboxylic acid esters, followed by sulfonation of double bonds or quarternization of amines. The resins are intended to increase the hydrophilic character and water absorption of the polyurethane coatings by reacting into the polyurethane backbone in an end-to-end fashion, rather than as pendent groups. Such resins which react in an end-to-end fashion, as opposed to remaining pendant at the end of the polyurethane chain, cannot provide for a clear delineation of hydrophilic and hydrophobic groups and in this respect do not behave as surfactants, i.e., they do not provide cooperation between distinct hydrophilic and hydrophobic portions to reduce interfacial tension. (See for example U.S. Pat. No. 3,822,238 to Blair et al.)
Polyurethane compositions have also been suggested which are useful as coatings for transparent substrates with improved self-healing properties and prevention against formation of surface moisture. The polyurethane compositions are prepared from a reaction of an isocyanate with a polyol mixture including a difunctional sulfonated polyether polyol and a trifunctional polyol. Such a polyurethane composition incorporates only polyol combinations which impart hydrophilic character to the coating, and does not further incorporate into the composition a surfactant material. (See for example U.S. Pat. No. 4,754,152 to Fock et al.)
However, these compositions do not provide permanent fog resistance properties, i.e. fog resistant properties which last after repeated washings or extended soaking in water, nor are they effective for more than a few hours of use.
Additionally, it is known to incorporate non-ionic surfactants containing reactive functional groups into polyurethanes prepared with polyvinylpyrrolidone as a hydrophilic agent. For example, anti-fog coating compositions incorporating an isocyanate prepolymer which is reacted with a polyvinylpyrrolidone polymer, the reaction product thereof being subsequently reacted with a non-ionic surfactant having reactive groups for reacting with the isocyanate, for instance, hydroxyl reactive groups are known. Polyvinylpyrrolidone polymers, however, while serving to increase the hydrophilicity of the polyurethane matrix and improve anti-fog properties, generally reduce the scratch-resistance, chemical resistance, water sensitivity, and durability of the cured polyurethane surface. Thus, although these compositions, when cured, have been known to provide anti-fog properties, their solvent sensitivity, flexibility and scratch resistance properties are less than desirable. (See for example U.S. Pat. No. 4,467,073 to Creasy)